1. Field of the Invention
The present invention generally relates to an image processing apparatus and an image processing method in which image data can be processed at high speed.
2. Description of the Related Art
Recently, documents have been digitized by reading the documents by, for example, a scanner. When the size of image data of a digitized document is large, it takes a long time to transfer, store, and reproduce the digitized document. For example, when a document of A4 size (210 mm×297 mm) is read by a scanner and image data of the read document are stored in a full-color bitmap format in which resolution is 300 dpi and each RGB pixel has 8 bits per color, the size of a storage unit must be approximately 25 MB. Consequently, when the image data are transferred and stored, a wide transferring band and a large amount of storage must be provided.
In order to solve the above problem, image data are compressed and the compressed image data are transferred and stored. As the compression method, JPEG (Joint Photographic Experts Group) compression can be used. However, in a case where the JPEG compression is used, when image data of a letter region (text region) having a rapid and complex density change are compressed, the compressed image data may not have sufficient reproducibility. In order to solve the problem, in a method, the image data of the letter region are separated from image data of a photographic region (non-letter region), suitable compression methods are applied to the corresponding image data of the letter region and the photographic region, and image data of a full-color document are reproduced while maintaining high quality of the letter region. As such as method, there is a compact PDF (portable document format) technology.
In Patent Document 1, image data are separated into image data of a letter region and image data of a line drawing region; the image data of the letter region are compressed by JPEG and the image data of the line drawing region are recursively compressed by MMR (MODIFIED MODIFIED READ) compression. In patent document 2, image data of a letter region are extracted and image data of a multilevel background region are superposed by using coordinates. In Patent Document 3, image data of a letter region and image data of a non-letter region are displayed by switching between them by using mask data in which the image data of the non-letter region are formed of white pixels.    [Patent Document 1] Japanese Patent No. 2611012 (Japanese Laid-Open Patent Application No. 3-104380)    [Patent Document 2] Japanese Laid-Open Patent Application No. 2006-238405    [Patent Document 3] Japanese Laid-Open Patent Application No. 2006-197178
However, in Patent Document 1, only a character separation coding method is disclosed. In addition, in Patent Documents 2 and 3, when a full-color document is reproduced with a small data size while maintaining high quality of the letter region, many image data processing processes are required and the processes need plenty of periods.